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Between 1914 and 1945 (with a final act in the Balkans in the 1990s), this led to world-historical disaster, mass exterminations, ruthless wars for mastery. But out of those conflicts came a new kind of hybrid order. The nations would have self-rule, within borders redrawn by war and ethnic cleansing. But they would be supervised by a kind of postmodern empire, an imperial bureaucracy without the emperor — the European Union.

The outlier, as always, was Great Britain. Like its rivals, the United Kingdom lost its overseas colonies, but it kept much of its domestic empire, the several nations —English, Scottish, Welsh and Ulster Irish —that still share a flag and crown. And as befits its anachronistic status, Britain has held itself somewhat aloof from the European Union's postmodern imperium, joining the union but not its common currency.

These distinctive arrangements have been good for the U.K. overall. Remaining a united kingdom has magnified its global clout, and being in the EU, but not fully of it, has spared it the worst of the continent's Euro-driven woes.

But neither arrangement may last much longer. In the headlines, last week's British elections were a big victory for David Cameron's Conservatives. But the deep winners were the forces of nationalism, Scottish and English, which suddenly have the United Kingdom as we know it on the ropes.

The Scottish story is the more remarkable one, since a decade ago Scottish independence still seemed a crank's hobby, and after losing last year's referendum on independence it was assumed that the nationalists had shot their bolt.

Instead, the referendum campaign seems to have affected Scottish politics more than its technical outcome. As Alex Massie, a pro-union Scottish writer, put it last week, in Scotland, "Nationalism is our new secular religion," and the politics of identity suddenly "defeats all comers." All but three, as it turned out: In Thursday's vote, the nationalists took 56 out of 59 parliamentary seats in Scotland, effectively turning the heathered north into a single-party state.

What the Scottish nationalists ultimately want is to trade the Union Jack for the EU's postmodern bargain: ethnically rooted self-government under a distant supranational umbrella, rather than a political union that can back wars or budget cuts that most Scots oppose. Not every Scot who voted nationalist last week is ready to support this vision. But the pull of union is clearly weakening in the north. Meanwhile to the south, a more English sort of nationalism wants out of the European Union completely, and suspects the Scots are getting too sweet a deal within the U.K. as it is. This is the spirit at work in UKIP, the populist, antiglobalist party that's taking votes from both left and right, and in the Tory base as well.

The two nationalisms, north and south, can feed on one another. However reluctantly, Cameron's government will have to give "little Englander" sentiment its due. (He's promised a referendum on Britain's EU membership for 2017). This will confirm the Scottish nationalists in their alienation, their desire to rule themselves alone.

On paper, the arguments against both disunion and a "Brexit" from the EU remain potent. The Scots really do reap significant benefits from union, and the nationalist vision of Scotland as a kilted Norway, oil-rich and social-democratic, is unlikely to survive contact with the realities of independence. Abandoning Britain's "this far, no further" approach to Europe and leaving the EU outright, meanwhile, would cede economic and political influence (to France, most likely) for uncertain gains.

But these are practical arguments, and sometimes politics needs something more. The nationalists of Scotland and England, in different ways, offer a vision of political community as an object of belief, an end unto itself. Against that kind of message, it's not enough to defend the present order bloodlessly, to say, "Yes, it's anachronistic to have a miniature empire in this day and age, but really the net benefits make it worth keeping."

Instead, you have to argue explicitly for a GREAT Britain. You have to invoke the United Kingdom's world-bestriding past, which the Scots no less than the English sustained and died defending, with something more than awkward embarrassment. You have to make a case to the Little Englanders that Britain's multicultural, Europe-facing present can keep faith with that past and not just bury it. You have to demonstrate that a liberal empire, no less than an ethnic homeland, can be something real and rooted — something felt in "the blood and guts," as Massie put it during the Scottish referendum, "the bone and marrow of our lives."

I'm a Yankee; this not my argument to make. But if our cousins can't find leaders who can make it, there won't be a Great Britain anymore.